1. Field of the Invention
The present invention relates to computed tomographic (CT) imaging, and in particular to weighting in helical cone-beam CT.
2. Discussion of the Background
As medical CT manufacturers produce scanners with increasing number of detector rows, there arises a need for a practical reconstruction algorithm that can handle the increasing cone angle. Recently an exact helical cone beam algorithm of the shift invariant FBP type (Katsevich algorithm) was proposed, suitable for 1-PI and 3-PI reconstruction. After that practical ways to implement Katsevich algorithm in medical CT scanners were investigated. Generally, exact helical algorithms use only data within the helical PI-intervals, or, equivalently, within the N-PI window [10-11], where N=1, 3, . . . , is the number of helical half-turns used in reconstruction. However from the practical point of view N-PI window weighting has the following properties:
1) Some measured data located outside the N-PI window is not used, which means extra dose to the patient.
2) All data within the N-PI window is used with the same weight; while it is beneficial to the noise reduction, it makes an algorithm more sensitive to patient motion and
imperfections of real data.
3) The N-PI reconstruction restricts the choice of the helical pitch. For example, pitches in the range of 0.75-0.85 are too fast to be used with the 3-PI window, and are suboptimal to use with the 1-PI window, since only a small fraction of data is utilized.
On the other hand, 2D fan beam redundancy weighting has the following advantages:
1) Easily adjusted to the helical pitch
2) Smooth transition from 0 to 1 makes an algorithm more stable to patient motion and imperfections of real data.